1. Field of the Invention
The present invention relates to a wind turbine blade and an associated wind turbine, in particular a partial-pitch wind turbine blade.
2. Description of Related Art
During wind turbine operation, significant loads are experienced at the root ends of the wind turbine blades as the blades rotate under operation from aerodynamic forces. Such fatigue loads produce considerable stresses and strains in the wind turbine structure, requiring significant design limitations regarding the strength of the materials used in turbine construction, reinforcement, etc. Accordingly, it is of interest to develop particular wind turbine designs which can reduce such loads, providing for reduced design limitations for the overall turbine construction.
One particular wind turbine blade construction is a partial pitch wind turbine blade. A partial pitch wind turbine comprises a plurality of wind turbine blades having inner and outer blade sections. The outer blade sections are pitchable relative to the inner blade sections, such that the output power of the wind turbine can be controlled to maintain rated power output for different wind speeds. Examples of partial pitch wind turbines include the Danish Nibe A wind turbine, and the MOD-2 wind turbine developed by NASA.
Similarly in the patent literature, partial pitch blades have been described. This includes German Application DE 91754 and U.S. Pat. No. 7,994,650 B2 and which discloses a partial pitch wind turbine with an inner blade section and an outer blade section pitchable relative to each other.
Common to these partial pitch wind turbine blades is that they have an aerodynamic profile that changes continuously along the blade even when the inner blade section and the outer blade section are aligned to the greatest possible.
That is that the partial pitch blade presents a single continuous blade profile when the outer sections are unpitched (providing a consistent full-span blade profile), and during high wind conditions the outer blade sections may be feathered to reduce extreme loads. However, partial pitch blades continue to experience significant fatigue loads during normal operation.